Kawakami cautions that both the timing and the duration of gene expression are crucial. If the genes were turned on for too long, abnormal limbs developed. And if the genes were expressed too late in development, new limbs were unable to grow.

Experts say the findings are exciting, but they caution that much remains to be done before new limbs can be grown in mammals. The studies took place in still-developing animals, whose cells are likely much more flexible when it comes to inducing regeneration, says Hans-Georg Simon, a developmental biologist at Northwestern University, in Chicago, who studies limb and heart regeneration. Even mammals, including humans, show some regenerative capabilities. Under some circumstances, children as old as five can grow a new fingertip if the wound is treated correctly. But that ability is lost as we age.

"This pathway is undoubtedly a critical one," says Simon. "But other unknown factors are probably needed to reactivate adult, fully differentiated tissue to reconstruct a new structure."

Badylak adds that regeneration in mammals will likely require inhibition of our normal immune response, which triggers inflammation at the site of a wound. None of the animals that can regenerate limbs show this type of immune response.